winnpc.c: add low-level connect_to_named_pipe() function.

This contains most of the guts of the previously monolithic function
new_named_pipe_client(), but it directly returns the HANDLE to the
opened pipe, or a string error message on failure.

new_named_pipe_client() is now a thin veneer on top of that, which
returns a Socket * by wrapping up the HANDLE into a HandleSocket or
the error message into an ErrorSocket as appropriate.

So it's now possible to connect to a named pipe, using all our usual
infrastructure (including in particular the ownership check of the
server, to defend against spoofing attacks), without having to have a
Socket-capable event loop in progress.

errsock.c

@@ -55,7 +55,7 @@ static const SocketVtable ErrorSocket_sockvt = {
     sk_error_peer_info,
 };
 
-static Socket *new_error_socket_internal(char *errmsg, Plug *plug)
+Socket *new_error_socket_consume_string(Plug *plug, char *errmsg)
 {
     ErrorSocket *es = snew(ErrorSocket);
     es->sock.vt = &ErrorSocket_sockvt;
@@ -73,5 +73,5 @@ Socket *new_error_socket_fmt(Plug *plug, const char *fmt, ...)
     msg = dupvprintf(fmt, ap);
     va_end(ap);
 
-    return new_error_socket_internal(msg, plug);
+    return new_error_socket_consume_string(plug, msg);
 }

network.h

@@ -264,8 +264,12 @@ char *get_hostname(void);
 /*
  * Trivial socket implementation which just stores an error. Found in
  * errsock.c.
+ *
+ * The consume_string variant takes an already-formatted dynamically
+ * allocated string, and takes over ownership of that string.
  */
 Socket *new_error_socket_fmt(Plug *plug, const char *fmt, ...);
+Socket *new_error_socket_consume_string(Plug *plug, char *errmsg);
 
 /*
  * Trivial plug that does absolutely nothing. Found in nullplug.c.

windows/winnpc.c

@@ -15,7 +15,7 @@
 
 #include "winsecur.h"
 
-Socket *new_named_pipe_client(const char *pipename, Plug *plug)
+HANDLE connect_to_named_pipe(const char *pipename, char **err)
 {
     HANDLE pipehandle;
     PSID usersid, pipeowner;
@@ -33,9 +33,10 @@ Socket *new_named_pipe_client(const char *pipename, Plug *plug)
             break;
 
         if (GetLastError() != ERROR_PIPE_BUSY) {
-            return new_error_socket_fmt(
-                plug, "Unable to open named pipe '%s': %s",
+            *err = dupprintf(
+                "Unable to open named pipe '%s': %s",
                 pipename, win_strerror(GetLastError()));
+            return INVALID_HANDLE_VALUE;
         }
 
         /*
@@ -46,16 +47,18 @@ Socket *new_named_pipe_client(const char *pipename, Plug *plug)
          * take excessively long.)
          */
         if (!WaitNamedPipe(pipename, NMPWAIT_USE_DEFAULT_WAIT)) {
-            return new_error_socket_fmt(
-                plug, "Error waiting for named pipe '%s': %s",
+            *err = dupprintf(
+                "Error waiting for named pipe '%s': %s",
                 pipename, win_strerror(GetLastError()));
+            return INVALID_HANDLE_VALUE;
         }
     }
 
     if ((usersid = get_user_sid()) == NULL) {
         CloseHandle(pipehandle);
-        return new_error_socket_fmt(
-            plug, "Unable to get user SID: %s", win_strerror(GetLastError()));
+        *err = dupprintf(
+            "Unable to get user SID: %s", win_strerror(GetLastError()));
+        return INVALID_HANDLE_VALUE;
     }
 
     if (p_GetSecurityInfo(pipehandle, SE_KERNEL_OBJECT,
@@ -63,21 +66,33 @@ Socket *new_named_pipe_client(const char *pipename, Plug *plug)
                           &pipeowner, NULL, NULL, NULL,
                           &psd) != ERROR_SUCCESS) {
         CloseHandle(pipehandle);
-        return new_error_socket_fmt(
-            plug, "Unable to get named pipe security information: %s",
+        *err = dupprintf(
+            "Unable to get named pipe security information: %s",
             win_strerror(GetLastError()));
+        return INVALID_HANDLE_VALUE;
     }
 
     if (!EqualSid(pipeowner, usersid)) {
         CloseHandle(pipehandle);
         LocalFree(psd);
-        return new_error_socket_fmt(
-            plug, "Owner of named pipe '%s' is not us", pipename);
+        *err = dupprintf(
+            "Owner of named pipe '%s' is not us", pipename);
+        return INVALID_HANDLE_VALUE;
     }
 
     LocalFree(psd);
 
-    return make_handle_socket(pipehandle, pipehandle, NULL, plug, true);
+    return pipehandle;
+}
+
+Socket *new_named_pipe_client(const char *pipename, Plug *plug)
+{
+    char *err = NULL;
+    HANDLE pipehandle = connect_to_named_pipe(pipename, &err);
+    if (pipehandle == INVALID_HANDLE_VALUE)
+        return new_error_socket_consume_string(plug, err);
+    else
+        return make_handle_socket(pipehandle, pipehandle, NULL, plug, true);
 }
 
 #endif /* !defined NO_SECURITY */

windows/winstuff.h

@@ -373,6 +373,12 @@ Socket *make_handle_socket(HANDLE send_H, HANDLE recv_H, HANDLE stderr_H,
 Socket *new_named_pipe_client(const char *pipename, Plug *plug); /* winnpc */
 Socket *new_named_pipe_listener(const char *pipename, Plug *plug); /* winnps */
 
+/* A lower-level function in winnpc.c, which does most of the work of
+ * new_named_pipe_client (including checking the ownership of what
+ * it's connected to), but returns a plain HANDLE instead of wrapping
+ * it into a Socket. */
+HANDLE connect_to_named_pipe(const char *pipename, char **err);
+
 /*
  * Exports from winctrls.c.
  */